Multi-path searching

ABSTRACT

A method of multi-path searching in a cellular network is provided. The method carries out a correlation process over a predetermined integration period in relation to a first pilot signal received in connection with a first cell of the cellular network to generate first multiple correlation measurements corresponding to a plurality of time delays in the signal, analyzing the first multiple correlation measurements in relation to the time delays to identify multi-path positions in the signal. Information of the multi-path positions over a first predetermined integration period is stored as a first set of candidate multi-path positions and compared with a second set of candidate multi-path positions derived from second multiple correlation measurements corresponding to the time delays over a second predetermined integration period in relation to the first pilot signal, so as to confirm or reject candidate multi-path positions and define modified candidate multi-path positions.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national phase application based onPCT/GB2004/000995, filed Mar. 8, 2004, and claims the priority of GreatBritain Application No. 0305561.3, filed Mar. 11, 2003, the content ofboth of which is incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to multi-path searching in cellular communicationsystems, especially for mobile handsets in cellular networks.

BACKGROUND OF THE INVENTION

A mobile handset has to be designed to accommodate multi-path signalsthat may be present in the received signal, and for this purpose itincorporates a multi-path searcher which identifies the strongest of themulti-path signals to use. In a W-CDMA system, the Common Pilot Channel(CPICH) contains a predetermined sequence of pilot bits spread withknown channelisation codes and scrambled with known scrambling codes,and the multi-path searcher makes measurements on the CPICH signal bycorrelating this against the known CPICH scrambled chip sequence toidentify the path positions and select a sub-set to use for decoding.The multi-path searcher consists of multiple correlators which eachprocess the same scrambling sequence and channelisation code to carryout a correlation measurement on the received signal after it has beendelayed by a different time delay at the input of each correlator, themultiple correlation measurements so generated being analysed inrelation to the time delays to identify the multi-path positions. Inorder to improve the accuracy and reliability in detecting pathpositions, the correlation process is extended over an integration timesufficient to identify the path positions above the noise floor of thesignal. However, an increase in integration time results in an increasein processor/ASIC size and power consumption, which in turn leads to anincrease in the manufacturing cost of the handset

In addition, there is a requirement in the W-CDMA standard that ahandset should be able to handle multi-path signals, as described above,in connection with multi-path CPICH signals corresponding to differentcells of the cellular network, and that multi-path measurements shouldbe made in relation to a predetermined number of cells should within apredetermined time, each cell being identified by its own scramblingcode and channelisation code. Thus, if a multi-path searcher is used tomake successive multi-path measurements in relation to multiple cells,the integration time for each is limited. Alternatively, if multiplemulti-path searchers are provided to handle the multiple cells inparallel, then the manufacturing cost and power consumption isincreased.

An object of the invention is to provide multi-path searching in such amanner as to mitigate the above problems.

SUMMARY OF THE INVENTION

According to the invention, a multi-path searcher is provided whichcarries out a correlation process over a predetermined integrationperiod in relation to a first pilot signal received in connection withthe first cell of a cellular network to identify the position ofmulti-paths in the signal, characterised in that information of saidmulti-path positions over a first predetermined integration period isstored as first candidate multi-path positions and compared with secondcandidate multi-path positions derived from multiple correlationmeasurements made subsequently by the multi-path searcher over a secondpredetermined integration period in relation to the first pilot signal,so as to confirm or reject candidate multi-path positions and definemodified candidate multi-path positions.

Any uncertainty in candidate multi-path positions as measured in thefirst predetermined integration period is resolved by the candidatemulti-path positions as measured in the second and subsequentpredetermined integration periods, thus shorter integration periods canbe used to reduce hardware size and cost, while an insufficiently lowlevel of uncertainty in the measurement of candidate multi-pathpositions can still be accomodated.

By spacing said successive predetermined integration periods apart,multiple sets of integration periods can be interleaved and the samemulti-path searcher can be used to determine the multi-path positions inrelation to the pilot signal received in connection with each ofdifferent cells. Thus, the multi-path searcher carries out a firstcorrelation process in relation to the pilot signal of each of differentcells in succession to determine first candidate multi-paths for each,and then carries out a second correlation process in relation to each ofthe pilot signals again to determine second candidate multi-paths foreach to confirm or reject the respective first candidate multi-paths anddefine modified candidate multi-path positions.

Thus the multi-path searcher is able to meet the standard required inmeasuring the multi-path positions of a predetermined number of cellswithin a limited time whilst still controlling the size of the hardwareused.

The invention will now be described by way of example with reference tothe accompanying drawings:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of a multi-path searcher as used accordingto the invention;

FIG. 2 is a schematic drawing showing the time delayed outputs ofmultiple correlators in FIG. 1;

FIG. 3 is a plot of the outputs of the correlators of FIG. 1 in responseto a typical CPICH signal, showing candidate multi-paths; and

FIG. 4 is a diagram showing the timing of successive candidatemulti-path measurements for different cells.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The multi-path searcher in FIG. 1 comprises multiple correlators C₀, C₁,C₂, C₃ . . . C_(N) which are all fed the same input signal generated bya cellular receiver RX. The input signal is fed to the correlators via achain of delay elements D which each introduce a delay Δ so that theinput signal is delayed to each of successive correlators C₀, C₁, C₂, C₃. . . C_(N) by a progressively increasing interval Δ, 2Δ, 3Δ . . . NΔ asshown in FIG. 2. In the first of multiple integration periods T_(P1),each of the correlators processes the input signal using a scramblingsequence for the common pilot channel CPICH of a first cell CELL 1 so asto make multiple correlation measurements M₀, M₁, M₂, M₃ . . . M_(N),which are analysed in a processor A as a time plot, shown in FIG. 3, todetermine multi-path positions in accordance with a reference L.

These multi-path positions are stored in a corresponding first store P₁.

It will be appreciated that the correlation process in each correlatorinvolves a de-scrambling, de-spreading and accumulation, of the bitstreams representing the input signal to produce one of the probabilitymeasurements M₀, M₁, M₂, M₃ . . . M_(N). The reference level L is set ata predetermined level corresponding to the expected threshold for theexistence of a multi-path. As shown in the example of FIG. 3, twosignificant peaks are detected, L1 above the level L and L2 just belowthe level L. Data of both of these peaks L1 and L2 are stored in thestore P₁ as candidate multi-path positions.

As the correlation process continues, the correlators C₀, C₁, C₂, C₃ . .. C_(N) next make multiple correlation measurements over a secondintegration period T_(P2) in relation to the CPICH channel of a secondcell CELL 2 to determine corresponding candidate multi-path positionswhich are stored in the store P2. The same correlation process then runsfor each of successive integration periods T_(P3), T_(P4) . . . T_(PS)in relation to the CPICH channel of 3rd, 4th . . . 5th cells CELL 3,CELL 4 . . . CELL S to determine corresponding candidate multi-pathpositions and to store these in respective stores P₃, P₄ . . . P_(S).

At this point, the multi-path detector has analysed the candidatemulti-paths in S cells over a time period ST_(P), assuming that theintegration periods T_(P1), T_(P2), T_(P3), T_(P4) . . . T_(PS) are allequal to T_(P). It will be appreciated that T_(P)=T_(INT)+NΔ, whereT_(INT) is the period over which each correlator operates to generate acorrelation measurement. Therefore, the period T_(INT), delay Δ andnumber of correlators N are all selected in relation to the number ofcells S to ensure that the multi-paths are determined for the requirednumber of cells S in a predetermined time ST_(P).

Having measured candidate multi-paths for each of the cells S in a firstmeasurement cycle T1, as shown in FIG. 4, the correlators C₀, C₁, C₂, C₃. . . C_(N) then repeat the whole process in a second measurement cycleT2 to determine a second set of candidate multi-path positions for eachcell which are also stored in the respective stores P₁ to P_(S) holdingthe first candidate multi-path positions. The first and second set ofcandidate multi-path positions stored in each store P₁ to P_(S) arecompared with one another by the processor A to produce a modified setof candidate multi-path positions, some candidate positions beingconfirmed and others being rejected. This process of modification ofcandidate multi-path positions to produce a modified set of multi-pathpositions with a higher probability of correctness, continues insuccessive measurement cycles. In this way, any lower level ofprobability accepted for determining individual candidate multi-pathpositions, perhaps because of a shorter than optimum period T_(INT), iscompensated for by subsequent repeat measurements.

To sum up, the present invention discloses a multi-path searchercomprising multiple correlators (C₀, C₁, C₂, C₃. . . C_(N)) which eachprocess a predetermined scrambling sequence and channelisation code tocarry out a correlation measurement on a received pilot signal after ithas been delayed by a different time delay (D) at the input of eachcorrelator. Multiple correlation measurements (M₀, M₁, M₂ . . . M_(N))are generated and analysed (A) in relation to the time delays toidentify the multi-path positions. First candidate multi-path positionsare identified over a first predetermined integration period (T_(P1))and are stored and compared with second candidate multi-path positionsderived from multiple correlation measurements made subsequently by themulti-path searcher over a second predetermined integration period(T_(P2)) in relation to a first pilot signal. The first and secondcandidate multi-path positions are confirmed or rejected to definemodified multi-path positions. The successive predetermined integrationperiods (T_(P1), T_(P2)) are spaced apart to accommodate one or moresimilar sets of successive integration periods in an interleaved manner,and the same multi-path searcher is used to determine multi-pathpositions in relation to the pilot signal received in connection witheach of different cells (CELL 1, CELL 2, . . . CELL S). Thus, themulti-path searcher carries out a first correlation cycle (T1) inrelation to the pilot signal of each of different cells in succession todetermine first candidate multi-paths for each, and then carries out asecond correlation cycle (T2) in relation to each of the pilot signalsagain to determine second candidate multi-paths for each to modify therespective first candidate multi-paths.

The invention claimed is:
 1. A method of multi-path searching in acellular network in which a multi-path searcher is provided whichcarries out a correlation process over a predetermined integrationperiod in relation to individual ones of a plurality of pilot signalsreceived in connection with respective cells of the cellular network togenerate first multiple correlation measurements corresponding to aplurality of time delays in the signal, analysing said first multiplecorrelation measurements in relation to said plurality of time delays toidentify multi-path positions in the signal, characterised in thatinformation of said multi-path positions over a first predeterminedintegration period is stored as a first set of candidate multi-pathpositions and compared with a second set of candidate multi-pathpositions derived from second multiple correlation measurementscorresponding to said plurality of time delays made by the multi-pathsearcher over a second predetermined integration period in relation toan individual one of the pilot signals, so as to confirm or rejectcandidate multi-path positions and define modified candidate multi-pathpositions, wherein the first predetermined integration period and thesecond predetermined integration period are substantially equal to oneanother, wherein said second set of candidate multi-path positions isderived from a repeat correlation process performed to derive said firstset of candidate multi-path positions, and wherein first and secondmultiple correlation measurements are obtained for respective pilotsignals in the plurality of pilot signals received in connection withthe respective cells, and the first multiple correlation measurements ofa second cell are obtained before the second multiple correlationmeasurements are obtained for a first cell such that the multiplecorrelation measurements for the first and second cells are interleavedwith one another.
 2. A multi-path searcher for multi-path searching in acellular network by carrying out a correlation process over apredetermined integration period in relation to individual ones of aplurality of pilot signals received in connection with respective cellsof the cellular network to identify the position of multi-paths in thesignal, characterised in that storage is provided to store informationof said multi-path positions over a first predetermined integrationperiod as a first set of candidate multi-path positions and to store asecond set of candidate multi-path positions derived from multiplecorrelation measurements made subsequently by the multi-path searcherover a second predetermined integration period in relation to anindividual one of the pilot signals, and a comparator provided tocompare said first and second candidate sets of multi-path positions soas to confirm or reject candidate multi-path positions and definemodified candidate multi-path positions, wherein the first predeterminedintegration period and the second predetermined integration period aresubstantially equal to one another, wherein said second set of candidatemulti-path positions is derived from a repeat correlation processperformed to derive said first set of candidate multi-path positions,and wherein first and second multiple correlation measurements areobtained for respective pilot signals in the plurality of pilot signalsreceived in connection with the respective cells, and the first multiplecorrelation measurements of a second cell are obtained before the secondmultiple correlation measurements are obtained for a first cell suchthat the multiple correlation measurements for the first and secondcells are interleaved with one another.
 3. A mobile handsetincorporating a multi-path searcher as claimed in claim 2.